I need to remove about a 20 foot section of 2" cast iron drain pipe to replace it with PVC. What is the best way to cut cast iron? My understanding is that sewer gases can be flamable, so I assume cutting it with a saw-zaw may create sparks that could be dangerous. Do I need to be worried about sparks?

Alternatively, I could probably remove sections of the cast iron pipe at the "fittings" (I don't know the technical term for them). How are cast iron fittings attached (threaded? welded? etc.), and it is just easier to try to remove them instead of cutting the pipe?

I'll attach the new PVC to the remaining cast iron with the rubber hubs, so I assume my cut (or removal of the fittings) does not have to be perfectly clean.

How to cut it depends on many factors, and there is no one easy and safest method, since easy is a relative term, and safest usually depends on how you handle the pipe while cutting it, not the method of cutting. Why do you have to replace that section of piping? Methane gas IN CONCENTRATIONS is combustible, the concentrations in a sewer is well below that, otherwise we would be having manhole covers being blown in the air all over the city.

Thanks for your reply. The reason I need to replace it is that at one of the fittings, the cast iron has dropped some, so that the next section of cast iron pipe is actually going a little uphill and does not fall correctly (to my eye at least). It is not leaking at the "dropped" section's fitting, but I think it needs to have an even fall to prevent problems in the future. The sink has only backed up once in the couple of years since we have lived there, but I am guessing that was contributing to the problem.

It is "uphill" enough that I would guess about two feet of drain could have a little standing water in it. Fortunately, it is just a sink drain, so there should not be any solid material in it unless the disposal does not do it's job.

But regardless, that's what got me thinking I need to repair/replace it.

A drain requires 1/4" fall per foot of length, but do not trust eyeballing it. That's why levels are made. Assuming you are correct and the line is sloped wrong, you may be on the right track to change some of the cast. However, you need to be certain that putting in a piece of PVC will correct the problem. The best way to cut cast iron is with a snap cutter. You can also use a grinder, but these are a bit dangerous. If the blade gets pinched in the cut, the tool with rip out of you hands and seriously injure you. I've never had success using a recip saw. You also need to be aware that cast iron pipe is very heavy. Even a short piece can break an arm or leg if it fall on you. If you get the CI cut out, then the PVC can be splice in using banded couplers. These are NOT the neoprene sleeves with hose clamps on each end. The band runs the entire length of the coupler. Working with CI is difficult and dangerous for a novice. You would be wise to consider a professional for this job. A plumber is cheaper than an ER and much less painful.

Thanks for the good advice. You are right. In the long run, some decisions to save a little money can cost you more.

I had planned to use metal straps to make sure the cast iron did not fall during the cut, but that stuff is still very heavy, and even if I don't cause damage to myself, I could cause damage to ductwork, etc. which would just result in more headaches. I still may try it since it is 2" CI and not 4", but I'll definitely be as careful as possible.